Melt the butter in a large sauté pan over medium-high heat and add the spinach. Sauté until wilted (if using fresh) and any excess liquid has evaporated. Add the garlic and season to taste with salt and pepper. Let the spinach cool completely, then chill before assembling the Wellingtons.
Preheat the oven to 400°F (200°C) and line a baking tray with parchment paper.
Assemble the Wellingtons. If using homemade puff pastry, divide the dough into four portions and roll out each one into an 8-inch (20 cm) square. If using store-bought sheets of prerolled pastry, trim each piece to an 8-inch (20 cm) square.
Divide half of the spinach among the pastry squares, mounding it in the middle. Lightly season the fish and place a fillet on top of the spinach. Spoon the remaining spinach on top of the fish. Bring the corners of the pastry together, pinch the seams to seal the parcels and place, seam side down, on the baking tray.
Brush each Wellington with the egg wash. Use the back of a paring knife to mark a pattern on top of each parcel, but do not cut through the pastry.
Bake for 20 minutes, until the pastry is a rich golden brown. Serve immediately.